This invention relates to a sheet-fed press, more particularly an inadequately fed sheet detector wherein the position of a sheet (paper to be printed) stopped by a front guide or gauge is detected for preventing inadequate feeding of the sheet.
A sheet feeding apparatus utilized in a sheet-fed press is constructed such that sheets piled up on a pile table are sequentially sucked and fed into a nib between a sheet feed roller and a tape roller, then sent to a feed board by a sheet feed tape mounted on a tape roller under tension, and one ends of the sheets are caught by grippers of a swinging device to transfer the ends to grippers of a impression cylinder.
With this construction, the position of the sheet end gripped by the swing gripper at the leading end of the feed board is important. More particularly, fluctuation of this position causes deviation of the printing position on the sheet, thus forming defective printed matters and when the amount of gripping is small, the sheet drops off. Accordingly, it has been the practice to provide a front guide of the leading end of the feed board to receive and stop the leading end of the sheet, thus neatly aligning thereof and to install a sheet detector at this position so as to prevent unsatisfactory printing or breakage of the sheet caused by the overrunning thereof or prevent contamination of the impression cylinder caused by so called set off an cylinder that is when a pressure is applied across a rubber cylinder and the impression cylinder in the absence of the sheet therebetween.
For example, according to the first embodiment of Japanese Utility Model Publication No. 12509/1964, a luminous element protected by a holder is secured to the lower surface of a head board to face upward, and a light receiving element is supported by an arm on the opposite side of the head board to oppose the luminous element. A transparent acryl resin plate is provided for the head plate for transmitting light from the luminous element to the light receiving element. A front guide is mounted on one end of the feed board to be reciprocatable in the direction of feed of the sheet.
With this construction, however, while the trailing end of a first sheet fed to the impression cylinder is still in the detector, when the leading end of the next sheet reaches, the detector would misoperate by judging that two sheets are passing. Moreover, the detector detects the trailing end of the first sheet before arrival of the second sheet, whereby the detector misoperates. For this reason, in order to eliminate such misoperations it is necessary to detect only after the trailing end of the preceding sheet has left the detector. This makes it impossible to detect at an earlier time. In other words, the detector can not follow a light printing speed.
According to the second emodiment of the utility model publication referred to hereinabove, the light receiving element is supported by a holder pivotally supported by a shaft to be moved away from the detector. With this construction, since the trailing end of a preceding sheet fed to the impression cylinder is maintained above the light receiving element it becomes possible to positively detect the presence or absence of the succeeding sheet irrespective of the trailing end of the preceding sheet. For this reason, it is possible to detect more quickly than the first embodiment. This construction, however, accompanies the following problems. More particularly, where an additional printing is made on already printed portions, the position of the additional printing is caused to vary due to variation in the rotating speed of the impression cylinder or in the position of a platen for the additional printing. To improve registering or to widen the printed area, or to prevent release of the grippers, the position of the head plate is changed in the forward and backward directions so as to prevent troubles caused by adjusting the extent of gripping and feeding of the sheet.
Since the light receiving element is located just above the luminous element when the front guide is displaced somewhat in the direction of sheet feed a desired quantity of the light would be supplied from the luminous element to the light receiving element so that the detection accuracy is impaired with the result that regardless of the fact that the sheet is in a normal range the inadequately fed sheet detector produces an output or the detector does not produce an output even though the sheet lies in an abnormal range.
Another problem lies in that, according to the second embodiment, a light receiver holder must be added to the prior art mechanism, thus complicating the mechanism located between the feed board and the printing unit.